Rubber-bonded sensor magnets are used in magnetic encoder for use at positions of encoders such as speed sensors, etc. Generally, NBR has been so far used as a binder for rubber-bonded magnets of wheel speed sensor, but in the case of application to the position of engine-driving system and its surrounding positions, the service atmosphere is at high temperatures such as about 130° to about 170° C., and consequently NBR, whose upper limit heat-resistant temperature is about 120° C., cannot be used at such positions.
Rubber applicable even to such high temperatures as about 130° to about 170° C. includes, for example, silicone rubber, fluoro rubber, acrylic rubber, etc., but the silicone rubber has an oil resistance problem, whereas in the case of the fluoro rubber the rubber flexibility will be considerably lost when magnetic powder is filled in the fluoro rubber at a high packing density, and the mixing proportion of magnetic powder is limited, so a high magnetic force is hard to obtain. The acrylic rubber has a considerably low rubber strength when the magnetic powder is filled at a high packing density, and thus fails to serve as a binder.
The present applicant have so far proposed a rubber composition for magnetic encoders, which comprises an ethylene-methyl acrylate copolymer rubber, magnetic powder, and an amine-based vulcanizing agent. Magnetic encoders vulcanization-molded from such a rubber composition has not only distinguished heat resistance, water resistance, and saline water resistance, but also has a distinguished magnetic force, because of the magnetic powder being filled in the rubber component at a high packing density. However, the rubber composition still has processability problems, such as occurrence of foaming in case of peroxide cross-linking agent at the time of vulcanization molding and a poor mold releasability in case of amine-based vulcanizing agent.
Patent Literature 1: JP-A-2004-26849